Conventional vertical cavity surface-emitting lasers (VCSELS) typically have two flat resonator cavity mirrors formed onto the two outer sides of a layered quantum-well gain structure, and are significantly limited in single spatial-mode output power, typically a few milliwatts. While greater optical power can be achieved from conventional VCSEL devices by using larger emitting areas, such a large aperture device is not particularly practical for commercial manufacture or use, and produces an output which is typically distributed across many higher order spatial modes. Several schemes have been proposed for increasing single-model output power from surface-emitting devices. One approach is to replace one of the mirrors adjacent the active region of a conventional VCSEL device with a more distant reflector whose curvature and spacing from the active region preferentially supports a fundamental spatial mode. Such a device architecture is called a VECSEL (vertical Extended cavity surface Emitting Laser).
“High single-transverse mode output from external-cavity surface emitting laser diodes”, M. A. Hadley, G. C. Wilson, K. Y. Lau and J. S. Smith, Applied Phys. Letters, Vol. 63, No. 12, 20 Sep. 1993, pp. 1607-1609, describes a triple-mirror, coupled-cavity device with an epitaxial p-type bottom Bragg mirror and undoped quantum-well gain structure grown on an external p-type substrate followed by an n-type coupled cavity intermediate mirror. The medium between the coupled cavity intermediate n-type mirror and the output coupler was air. Since any heat produced in the active gain region must be removed through the relatively thick p-type substrate, the practical output power from such a device is limited to about 100 mw for pulsed operation and to only a few mw for continuous (“cw”) operation.
“Angular filtering of spatial modes in a vertical-cavity surface-emitting laser by a Fabry-Perot étalon,” by Guoqiang Chen, James R. Leger and Anand Gopinath, Applied Physics Letters, Vol. 74 No. 8, Feb. 22, 1999, pp. 1069-1071, describes an integrated Fabry-Perot étalon formed of GaAs between a reduced bottom mirror stack of the VCSEL and a backside dielectric mirror, to thereby form an integrated coupled oscillator in which the angular plane-wave spectra of the higher-order modes have been spatially filtered out. No electrode configurations are shown or described and it is not apparent how that device could be electrically excited to produce high levels of output power.
My commonly assigned PCT publication WO 98/43329 describes a novel architecture for an electrically-excited vertical extended cavity surface emitting laser (VECSEL) device that enables the output power emitted in the single, lowest order TEM00 spatial mode to be scaled upwards more than an order of magnitude beyond that achievable with other known VECSELS, while being much more practical and manufacturable than was previously achievable. In that device, the quantum-well gain layers were grown directly on the bottom surface of the n-type substrate; this growth was then followed by the usual highly-reflecting p-type DBR cavity mirror. The laser cavity was formed by depositing an anti-reflective coating on the top surface of the n-type substrate, and placing a concave external mirror away from the substrate with the mirror's optical axis oriented perpendicular to the plane of the substrate, such that the n-type substrate was located physically and optically within the laser cavity. Such an internal substrate configuration not only provides structural integrity and ease of manufacture (especially when the external mirror is formed on or otherwise placed directly on top of the inverted substrate), it also facilitates an electrode placement that is optimal for efficient electrical excitation and operation in the TEM00 mode with a larger aperture and high output power levels than would otherwise be possible. However, especially in an electrically pumped device with a relatively thick substrate inside the laser cavity, increasing the doping of the substrate (desirable to minimize carrier crowding and electrical resistance) also increases the optical loss at the laser wavelength and the overall efficiency of the device is correspondingly reduced.